Cephalosporin antibiotics are markedly effective drugs for the treatment of infectious diseases in mammals because of their excellent antibacterial actions and low toxicity against mammals. In recent years, a number of cephalosporin derivatives having an aminothiazolyl-.alpha.-(substituted or unsubstituted)hydroxyiminoacetyl group at the 7-position of the cephem ring have been studied and developed because of their potent antibacterial activities and high stability against .beta.-lactamase.
So-called "third generation cephalosporin antibiotics" such as cefotaxime, cefmenoxime and the like are used frequently in most countries of the world, which are characterized by the presence of an aminothiazolyl-.alpha.-(substituted or unsubstituted)hydroxyiminoacetyl group at the 7-position of each compound and by their potent antibacterial activities with a broad range of antibacterial spectrum. However, certain compounds of these third generation cephalosporin antibiotics such as cefotaxime, cefmenoxime and the like are still imperfect in terms of their antibacterial activities upon Pseudomonas aeruginosa and methicillin-resistant mutants of Staphylococcus aureus (hereinafter, referred to as "MRSA") which have recently been causing clinical problems. Especially, MRSA has in recent years been causing trouble of triggering serious bacterial infection-related diseases. In consequence, great concern has been directed toward the development of novel cephalosporin antibiotics having improved anti-bacterial activities upon this methicillin-resistant bacterium.